1. Field of the Invention
The invention relates to a device for long-term leakage monitoring, in particular for flat tank structures comprising a sensor cable arranged on the outer surface of the tank casing, a collection chamber which, with respect to its height, is arranged at least partially below the bottom of the flat tank structure, surrounding its circumference preferably in an annular manner, and which via an inclined cantilever is connected with the lower end of the outer surface of the tank casing, with the sensor cable being arranged on the cantilever or in the bottom area of the collection chamber.
2. Description of the Related Art
From the German Patent DE 43 22 859 C2, a device for long-term leakage monitoring of double-wall vessel systems, in particular of double-wall tank bottoms, is known.
The device shown therein makes it possible to cyclically or simultaneously monitor with little expenditure a plurality of double-wall tank bottoms with respect to the occurrence of leakage. It can be determined by means of this device whether a leak has occurred on the outer bottom or in the intermediate bottom, whereby it is possible to determine the size and location of the leak. For this purpose, it is proposed therein to attach an intermediate layer at the inner side of an outer bottom, with the layer serving to support the inner part of a double-bottom. A sensor cable with a variable dielectric constant is installed within the intermediate layer in a protective manner. In the case in which a liquid enters the monitored space, the dielectric constant of the sensor cable changes so that the presence of leakage can be determined by means of an appropriate evaluation device.
By means of the special liquid-selective sensor cable, both a leak in the inner space as well as in the outer bottom can be detected. The detection of leaks in the outer casing of the tank, however, still represents a problem. Any leakage which might be present can only be determined visually and only above a certain leakage flow volume. There are also problems if the double bottom has been installed later, e.g. by lining with synthetic material.
DE 39 37 638 A1 proposes to form collection troughs which might comprise a hollow space in order to arrange a monitoring sensor in said hollow space. The collection troughs form the supporting surface for flat tank structures provided thereon so that any escaping liquids from leaks which might occur in the outer casing can also be collected and disposed of without constituting an environmental hazard.
However, problems will also arise if, due to a high surface pressure and thermal alternating stresses, cracking in the trough results. Though existing cracks can be detected by the sensors provided in the hollow spaces upon the ingress of liquids, e.g. environmentally hazardous chemicals, the actual purpose of the collection troughs, however, is no longer ensured in this case.
Finally, under the consideration of the teaching according to DE 39 37 638 A1, it is not possible to correspondingly retrofit already existing tank farms in accordance with environmental protection aspects without extensive disassembly work.
In order to be able to monitor vessels which, for example, contain hydrocarbons, in particular, gasoline, it has been proposed according to DE 33 36 335 A1 to arrange a sensor line at the outside of the vessel wall. The known sensor line comprises a sheathing which is soluble in hydrocarbon and is connected with an evaluation section.
In the case in which, according to the above described known solution, a leak occurs in the outer casing, it is assumed that, due to gravity, escaping liquid runs down the outer wall and comes into contact with the sensor cable arranged at the tank outer surface. In this case, however, the following problems arise. On the one hand, the liquid volume which escapes through the leak cannot be collected easily, and, on the other hand, there is the risk that in the case of major leaks, the liquid stored in the tank, which leaves the tank laterally and in a spraying manner, so that the sensor cable either does not come into contact with the liquid, or comes into contact only insufficiently, or too late, so that an existing leak is not signalised at all or only with a delay.
According to EP 0 783 100 A2, an advanced device for long-term leakage monitoring of tank casings was proposed with a special collection chamber being provided at least partially below the bottom and laterally of the flat tank structure. The known collection chamber can, for example, be formed by an annular pipe which is located below the bottom of the tank, circumferentially surrounding same. The collection chamber is connected with a special cantilever which has been retrofitted to the flat tank structure, with the cantilever having a gradient towards the collection chamber. The cantilever and the, for example, annular pipe form a collection chamber or a collection volume, respectively, which is capable of collecting escaping environmentally hazardous liquids until the drainage of the tank and/or the elimination of the leak, even in the case of major leaks. The cantilever and the collection chamber or the annular pipe, respectively, or the rim of a trench, which together with the cantilever form the collection chamber, are provided in such a manner that a sufficient distance from the tank casing is given so that in the case of a major leak, greater liquid amounts which escape laterally can be collected and come into contact with a sensor cable. The sensor cable itself is arranged in the area of the cantilever or in the bottom area of the collection chamber, e.g. in the bottom of the trench or of the annular pipe. Due to the fact that the sensor cable is made to be liquid selective, entering rain water, for example, will not falsely result in activating a leakage alarm. The solution shown in EP 0 783 100 A2, whose disclosure content is to be explicitly assigned to the subject matter of the application, as will be described in the following, enables a simple retrofitting of existing flat tank structures in an advantageous manner so that it is possible to respond to modified legal regulations of, for example, the Water Resources Management Law. An immediate advantage is that the otherwise necessary large area sealing of the base surfaces of flat tank structures can be omitted so that a natural return flow of rain water is ensured. The problem of a fire hazard, however, is still unsolved, in particular with respect to so-called tanks of Class A1.
In the case where escaping liquids in combination with the ambient air form an explosive mixture or, where due to the properties, there is a risk of self-ignition, probably resulting from an atmospheric discharge, there are considerable risks for tank farms. When, upon the detection of the fire, fighting of the fire is done by means of the usual agents, there is the risk that leakage liquid reaches water-carrying layers and thus the ground water due to large amounts of water or fire extinguishing foam to be used.
It is therefore the object of the invention to provide a device for long-term leakage monitoring, in particular, of flat tank structures, which makes it possible to increase the safety in the operation of such that tanks or of a plurality of flat tank arrangements, without leaving escaping liquid undetected for a long time, or, in the case of a fire, without fire extinguishing agent in combination with leakage liquid draining off in an uncontrolled manner.
The object of the invention is solved by a fire signaling sensor being located above the collection chamber which, in the case of fire, in cooperation with an extinguishing supply, enables the activation of the automatic forming of the collection chamber, and the collection chamber is a pipe or a pipe segment which surrounds the flat tank structure in an annular manner, comprising inlet slots or inlet holes facing towards the cantilever, with a circumferential collar plate being arranged on the pipe for the formation of in distinguishing agent retention chamber. Disclosed is also the monitoring of a double-wall tank bottom in a particularly advantageous manner, with the double-bottom construction having a glass fibre reinforced layer sequence of synthetic material. According to the invention it is proposed to installed a liquid selective sensor cable within the spacing fabric of the layer sequence so that flat tank structures which have been retrofitted or equipped in such a manner can be monitored with respect to their tightness.
According to an essential basic idea of the invention the tank casing leakage monitoring is further developed by means of an annular collection chamber in such a manner that a signalling fire sensor is arranged above the collection chamber. In the case of a fire, this signalling fire sensor, in cooperation with a fire extinguishing agent supply, e.g. a surrounding fire extinguishing pipe with extinguishing agent outlet openings, enables the activation of the automatic foaming of the collection chamber. By a defined amount of the extinguishing agent, which corresponds to the maximum possible extinguishing agent retention volume, an undesired escape of a mixture of leakage liquid and extinguishing agent into the environment can be prevented.
The inventive extinguishing retention chamber is created by an additional circumferential collar plate which is joined with the pipe or pipe segment which forms the collection chamber and surrounds the tank in an annular manner. Above the collar plate an also circumferential extinguishing foam line with extinguishing agent outlet openings is arranged which, upon the detection of a fire by the fire signalling sensor, allows a directed supply of extinguishing agent into the collection or extinguishing agent retention chamber.
The above described solution makes it possible to meet the requirements of both water protection and fire protection, in particular, with respect to Class A1 tanks. Foaming is effected analogously to the specifications of DIN 14493.
The sensor cable which is arranged at the cantilever or in the bottom area of the collection chamber has hydrocarbon selective properties. In order to achieve a directed drainage of the collection chamber including the extinguishing agent retention chamber, the collection pipe comprises at least one drainage means which is provided with a controllable shut-off means.
In order to increase the effectiveness and for a faster response, the sensor cable is arranged at the end of the cantilever facing the collection chamber and in a manner designed to retain any liquid.
The gradient of the cantilever compensates the height difference between a tank bottom or, respectively, tank or circular foundation relative to the portion of the collection chamber which is on the surrounding level. In one embodiment, the inlet slots of the collection pipe are preferably formed as longitudinal slots.
Instead of a pipe shape, the collection chamber surrounding the flat tank structure may be formed by a rim or trench starting from the end of the cantilever remote from the tank, with the drainage means with shut-off capability being arranged on the rim.
The annulus formed by the pipe or the rim or the trench, respectively, has a diameter which is between 1.5 and 2.5 times the diameter of the flat tank structure so that under consideration of the circumferential collar plate an adequate collection volume for escaping fluid or extinguishing agent, respectively, can be ensured.
As with the known state of the art, the sensor cable is a coaxial cable comprising a dielectric which changes reversibly as a function of the liquid effect, with the liquid or humidity effect being able to be demonstrated by the evaluation of high-frequency pulse echoes.
In order to achieve easy assembly and disassembly, the extinguishing foam line is removably attached at the collar plate. The preferred type of attachment is by means of pipe clamps. At the same time, these pipe clamps can be used for securing the fire signalling sensor, in particular, a fire signalling sensor cable.
With the inventive solution it is achieved that in the case of a major leak, greater amounts of liquids which escape in a lateral direction can be collected and come into contact with a sensor cable. At the same time, fire monitoring and fire fighting of the leakage collection chamber is provided via the fire signal-carrying cable or the fire signalling sensor, respectively.
Existing flat tank structures can be retrofitted with the inventive device in a simple manner, in that they are provided with a cantilever and the other means for forming the collection chamber or the extinguishing agent retention chamber, respectively. Thus, the pertinent legal regulations, e.g. of the Water Resources Management Law, can be fulfilled with minimum expenditure. The use of the inventive device increases the safety in the storage of liquids which are a hazard for water, and it is excluded that, in the case of a leak or even of a fire escaping, environmentally hazardous liquids result in the pollution or a detrimental change of inshore waters or the ground water. The otherwise necessary large area sealing of the base surfaces of flat tank structures can be omitted.